Shift+Enter no longer inserts newline in prompt

Bug Description

Shift+Enter no longer inserts a newline in the Claude Code prompt input. Pressing Shift+Enter now behaves the same as Enter (submits the message) instead of inserting a newline.

This used to work correctly in previous versions.

Environment

• Claude Code: 2.1.70
• OS: macOS 26.2
• Terminal: VS Code integrated terminal (1.109.4)
• Shell: zsh
• tmux: 3.4 (Claude Code runs inside tmux)
• Node: v22.14.0
• No custom keybindings configured (~/.claude/keybindings.json does not exist)

Steps to Reproduce

1. Open Claude Code in VS Code integrated terminal (inside tmux)
2. Type some text in the prompt
3. Press Shift+Enter to insert a newline
4. Expected: A newline is inserted in the prompt
5. Actual: The message is submitted instead

Additional Context

• No conflicting tmux key bindings for Shift+Enter
• Alt+Enter behavior is also the same (submits instead of newline)
• No custom keybindings in ~/.claude/settings.json